262 PRACTICAL ORGANIC CHTO'MISTRY
the anhydrides of hexahydrophthalicacid, they are distinguished
as cis- and trans-compounds (see Notes on Prep. 37, p. 265).
NCH.CO HoC' CH.CO
Ethyl Tartrata—The speculations of Pasteur (1860) on the
cause of the optical activity and hemihedry of tartaric acid and
its salts, and of Wislicenus (1873) on the existence of three
lactic acids, have developed in the hands of Van't rioff and
Le Bel (1874) into the present theory of stereo-chemistry or
atomic space arrangement. Optical activity is found to be in-
variably associated with the presence in the substance of an
asymmetri.-:. carbon atom, i.e. one linked to four different groups.
Now every asymmetric (unsymmetrical) object like a hand or
foot has its fellow ; but the two do not precisely overlap, and
every substance containing an asymmetric carbon atom, round
which the four groups are distributed, not, as usually repre-
sented, in one plane, but in space of three dimensions, is
capable of existing in two forms, which correspond to a left and
right hand, or to an object and its reflected image.
This is represented by making- the carbon atom the centre ot
a tetrahedron and attaching the four different groups to the
four solid angles. The two forms will then appear as in the
Fig., in which ABCD represent four different groups. When